1. Field of Invention
The invention relates to an auto-focus (AF) method, and more particularly, to an AF method to automatically switch between different focus value searching algorithms.
2. Background of the Invention
Digital single lens reflex cameras (also named DSLR) are characterized by delicate and complicated mechanical structures as well as enhanced functionality and maneuverability. In addition to shooting skills of users and environmental factors, the built-in auto-focus (AF) systems of the DSLR also pose a significant impact on the quality of photos.
Generally, according to the AF technology, the lens of the DSLR is moved to adjust the relative distance between the lens and an object to be shot. In response to different locations of the lens, focus evaluation values (hereinafter “the focus values”) of a subject frame having the to-be-shot object may be respectively calculated until the peak focus value is acquired. The peak focus value represents the location of the lens where the subject frame may have the optimal sharpness. Nonetheless, in order to obtain the optimal sharpness, the lens is required to move back and forth, and relevant calculations are continuously made. This thus leads to significant time consumption. In consideration of focus speed, DSLR often obtains a local peak through adopting a fast AF technique, i.e., a mountain-climbing method. In this case, however, the autofocus may fail if a near object and a far object co-exist in the subject frame.
For instance, FIG. 1 is a schematic diagram illustrating a focus value curve of an image. Here, the horizontal axis represents the lens-movement steps, and the vertical axis represents the focus value. FIG. 2(a) and FIG. 2(b) are schematic diagrams respectively illustrating a display image on a DSLR while the DSLR is focusing a scene. The focus value curve C1 shown in FIG. 1 indicates calculations made on the subject frame as shown in FIG. 2(a), for instance. According to the subject frame shown in FIG. 2(a), the to-be-shot object includes a notebook computer, a mouse, and a cup at a distance, and the focus frame 201 only contains one focus target (i.e., the notebook computer). As illustrated in FIG. 1, at the location of the lens corresponding to the lens-movement steps S1, the local peak F1 may be obtained, i.e., the location of the lens allows the subject frame to have the optimal sharpness. However, with reference to FIG. 2(b), after the AF procedure is successfully performed, another object (a pen) to be shot is also moved into the focus frame 203 for performing the AF procedure. At this time, the focus frame 203 contains the relatively far notebook computer and the relatively near pen. Since the AF procedure is already successfully performed on the notebook computer, the local peak may still be searched first from the locations of the lens corresponding to the lens-movement steps S1 shown by the focus value curve C1 when the AF procedure is performed once again. Namely, in the focus frame 203, the DSLR still focuses the relatively far notebook computer, and thus the autofocus on the relatively near pen fails. In view of the above, how to develop an AF method that may achieve favorable efficiency and satisfactory accuracy is one of the issues to be resolved.